KY Hall, reed, temperature and sound boards

The 37-in-1 kit's four-pin LM393 boards: the KY-024 Hall, KY-025 reed, KY-028 thermistor and KY-037 sound modules. DO goes high on detect.

The four-pin boards of the 37-in-1 sensor kit: the KY-024 linear Hall sensor, the KY-025 reed switch, the KY-028 digital temperature sensor and the KY-037 big sound sensor. Aliases: linear magnetic Hall module, reed module, thermistor module, high-sensitivity sound module.

Each is the sensor on an LM393 board with a blue multi-turn trimmer: the sensor comes out on AO, the comparator compares it with the trimmer, and DO goes high and lights the second lamp when the sensor is past it.

Pins

Pin	What it does
AO	The sensor's voltage, printed A0.
GND	G.
VCC	+, 3.3 V to 5 V.
DO	High, with the lamp lit, when the sensor is past the trimmer.

The header order varies between batches; this is the one printed on most, A0 G + D0. Read your board's silk screen.

The sensors

KY-024. A 49E linear Hall sensor, the same model as the bare 49E: half the supply with no field, 1.4 mV a gauss on 5 V, up for a south pole at the face. DO trips only for the pole that lifts AO. threshold is 0.6 by default, 0.5 V above rest, about 360 G, which the magnet makes at about 7 mm.

KY-025. A normally open reed under a 10 k pull-up, so AO is two levels: the supply with no magnet and nearly ground with one. It is a switch, not a measurement. It closes at close mm (15) and opens past close / release (18.75).

KY-028. A 10 k NTC bead (beta 3950, beta) as the lower half of a divider under 10 k, so AO falls as it warms: half the supply at 25 C. DO goes high when it is warmer than the trimmer; threshold 0.45 trips at about 30 C, a finger on the bead.

KY-037. The KY-038 board with the bigger capsule, a CUI CMA-6542PF, -42 dB re 1 V/Pa on its sheet: 2 dB more sensitive, and still no amplifier. A clap pulses DO; talking barely moves AO.

The reed's pull-up and the top of the thermistor's divider are taken as 10 k, the value the listings give. Where a new board's trimmer sits is anyone's guess; the defaults are set so the obvious thing (a magnet, a finger, a clap) trips it.

What it does not model

The reed's fraction of a millisecond of bounce, the bead's thermal lag, and the comparator's lack of hysteresis, which makes a real DO chatter right at the trimmer.

Common mistakes

Reading the KY-025's AO as a field strength. It only says closed or open.

Reading the KY-028 as a thermometer from DO. DO is a thermostat; read AO and work the temperature out from the bead's beta for a number.

See it in action

KY comparator boards and an IR switch reads each D0 on pins 2 to 5 and prints what it sees. Open it at /templates.